1. Field of the Invention
The present invention relates to power amplifier circuits and radio frequency transmitter systems, and particularly to an augmented twin nonlinear two-box modeling and predistortion method for power amplifiers and transmitters that provides predistortion suitable for highly nonlinear power amplifiers and transmitters having memory effects.
2. Description of the Related Art
Energy efficient wireless communication systems are being sought as part of the global concern for greener communication systems. Most of the energy savings can be made at the network level, as well as on the base station side. In this context, improving the efficiency of the radio frequency (RF) power amplifier (PA) is perceived as a highly attractive alternative that can enable higher efficiency transmitters and greener communication systems. Amplifiers with efficiency figures in the 70% to 80% range are being designed, although their use for wireless communication applications and their field adoption are still conditional on their ability to meet the linearity requirements of wireless communication standards. Indeed, achieving high efficiency amplification comes at the expense of severely nonlinear behavior due to the inherent efficiency-linearity dilemma in power amplifiers. Since linearity is a must, power efficient amplification circuits are always used together with a linearization technique that allows for mitigating nonlinear distortions of power amplifiers operating in their power efficient nonlinear region. For modern base station applications, baseband digital predistortion technique is the preferred linearization method, since it allows for acceptable linearity levels with a continuously increasing modulation bandwidth capability. One major advantage of baseband digital predistortion is its high flexibility and reconfigurability due to the digital implementation of the predistortion function and the availability of a wide range of digital processing functions that can compensate for static and dynamic distortions.
With the large adoption of baseband digital predistortion-based linearizers, behavioral modeling of RF power amplifiers and transmitters has received increasing interest, mainly motivated by the need to accurately predict their nonlinear behavior for system level simulations, and especially because predistortion can be perceived as a reverse behavioral modeling problem. A large variety of single-box and two-box structures have been reported for the modeling and predistortion of RF power amplifiers and transmitters. Single-box models range from the comprehensive and computationally heavy Volterra series to the compact memory polynomial model and its variants, such as the envelope memory polynomial, the orthogonal memory polynomial, the hybrid memory polynomial-envelope memory polynomial, the generalized memory polynomial model, and the like. Single-box models often result in a large number of coefficients when used for highly nonlinear RF power amplifiers and transmitters driven by multi-carrier wideband signals. In such cases, two-box models appear as a valuable alternative to maintain modeling performance, while requiring a lower number of model coefficients. Popular two-box structures include the Wiener, Hammerstein and their augmented versions, and the twin-nonlinear two-box models.
In modern applications, gallium nitride (GaN) based Doherty power amplifiers are used with multi-carrier wideband long term evolution (LTE) signals. GaN transistors offer superior performance compared to their laterally diffused metal oxide semiconductor (LDMOS) counterparts. However, they typically result in stronger memory effects. These effects get even stronger when advanced amplifier circuits, such as Doherty amplifiers with harmonically tuned carrier and peaking amplifiers, are used. Such a combination of device under test (GaN based high efficiency amplifiers) and operating conditions (multi-carrier LTE drive signals) requires the development of advanced behavioral models. These models are expected to either outperform state-of-the-art existing models, while requiring a comparable number of coefficients, or to achieve similar performance as their state-of-the-art counterparts, while requiring a lower number of coefficients.
Thus, an augmented twin nonlinear two-box modeling and predistortion method for power amplifiers and transmitters solving the aforementioned problems is desired.